Project Summary/Abstract This application is responsive to PA-18-817: Administrative Supplements for Research on Dietary Supplements (Admin Supp) Adequate vitamin D in early life is essential for healthy bone formation and other health outcomes, and the American Academy of Pediatrics (AAP) recommends a minimum daily intake of 400 IU of vitamin D for all infants and children. Many infants and children require vitamin D supplementation to meet these requirements, yet research indicates that the prevalence of vitamin D supplement use among U.S. infants and children is low. The impact of supplementation on reducing prevalence of deficiency and risk of adverse outcomes has not been well-studied in U.S. populations. We propose to examine the impact of early vitamin D supplementation and interaction with dietary vitamin D intake on plasma 25-hydroxyvitamin D (25(OH)D) levels in early childhood, and to assess the relationships of early vitamin D supplementation with bone health and body composition measured in mid-childhood and early adolescence. We will conduct this study within Project Viva, a well-established NIH-supported cohort of children enrolled in eastern Massachusetts, where the prevalence of vitamin D deficiency is high, particularly in the winter months. We will leverage existing longitudinal data on infant and child diet and supplement use and later measures of body composition and bone health, and will measure 25(OH)D levels in existing plasma samples collected from Project Viva participants in early childhood. This proposed supplemental project will allow us to examine one modifiable potential determinant of body composition trajectories observed through early adolescence, which is a key aim of the parent project ?Pre- and peri-natal predictors of childhood obesity? (R01 HD034568). This is a cost-effective opportunity to leverage existing data and plasma samples to study the impact of using widely accessible dietary supplements of vitamin D to mitigate the impact of inadequate vitamin D levels common in U.S. children.